Young spontaneously hypertensive rats (SHR) were either treated with hydralazine or hexamethonium or splanchnectomized, so that development of hypertension was effectively arrested for two weeks. The rate of incorporation of H3-lysine into noncollagenous proteins in vivo of the heart, aorta and mesenteric arteries was determined in the treated SHR as well as in control SHR and in normal Wistar/Kyoto (WK) rats. The lysine incorporation into the noncollagenous protein of mesenteric arteries was increased in 8-week-old SHR as compared to that of WK rats. The elevated lysine incorporation in the SHR was abolished by treatment with hexamethonium or by splanchnecotomy, but was not affected by treatment with hydralazine. It is suggested that neutral innervation is important for the increased synthesis of vascular noncollagenous protein during the early hypertensive phase in the SHR.